The “30-inch” brace is not 30 inches. It is 42 7/16.
In a jointed frame, a brace called a “30-inch brace” is not 30 inches long. Thirty inches is the length of the SIDES of the right triangle. The brace is the hypotenuse: 42 7/16 in. √(30² + 30²) = 42.43.
Beemer puts it plainly: an error of ⅛ in on a brace can keep a frame from going together. Not “make assembly harder”. Keep it from going together.
And that is not the only dimension where it matters. Reading his book from the page images rather than from machine-recognised text turned up three wrong figures: a tenon cut “¼ in” under nominal instead of ⅛ (tenons half as long as they should be), a post foot at “1¾ in” instead of 1⅞, and that brace given two different lengths in the same place — both wrong.
A generator does not get the fraction wrong. It sets the brace at 42 7/16 in because √2 is what it is, it dimensions from the shoulders, and it refuses to draw a joint that does not close.
Source : beemer-timber-frame-square-rule.md §6